This invention relates to turbine airfoils, especially turbine compressor airfoils, having improved impact and erosion resistance. This invention further relates to a method for forming improved erosion resistance coatings on these turbine airfoils.
Airfoils used in gas turbine engines can suffer from erosion and impact caused by particles ingested into the engine, especially in helicopter turboshaft engines. This is particularly true of the airfoils that comprise the turbine engine compressors. The effects of such ingestion can result in power loss, increased fuel consumption, increased gas turbine generator temperatures, as well as scrapping of compressor components long before the expected fatigue life limits. This performance loss can be sufficient to force these engines to be removed from the aircraft for compressor overhaul to regain lost performance.
Turbine compressor performance can be degraded because of impact damage to the leading edges of the compressor airfoils, as well as erosion of portions of the side or surface of the airfoil beyond the leading edge. Erosion and impact damage to airfoils can occur relatively quickly in desert environments due to sand ingestion. The impact of large sand particles can cause burr formation where the leading edge of the airfoil gets rolled or curled over, thus disturbing the airflow and degrading compressor performance. Additionally, erosion on the pressure surface or side of the airfoil contributes to early replacement or removal as the effective surface area of the airfoil decreases and the cross section (i.e., thickness) becomes too thin.
Both impact and erosion resistance needs to be addressed to increase the durability and longevity of gas turbine compressors, especially in environments such as the desert where particle ingestion is a significant issue. However, impact and erosion damage is the result of different problems created by the ingestion of these particles. Impact damage is primarily caused by high kinetic energy particle impacts on the leading edge of the airfoil. The particle flow, traveling at relatively high velocities, strikes the leading edge or section of the airfoil at a shallow upward angle of from about 30° below the concave or pressure surface or side of the airfoil, to angle directly or head on to leading edge (0°), i.e., at an angle perpendicular or 90° to the leading edge of the airfoil. Because the airfoil is typically made of relatively ductile metals, this shallow upward to direct or head on striking of the leading edge is what causes burrs to form where the portion of the leading edge struck by the particle deforms and then rolls over or curls. In addition to disturbing the airflow and degrading compressor efficiency, these burrs constrain the airflow, necessitating the engine to compensate by consuming more fuel for the required power.
Erosion damage is primarily caused by glancing or oblique particle impacts on the concave or pressure side or surface of the airfoil, and tends to be focused in the area in front or forward of the trailing edge, and secondarily in the area aft or beyond the leading edge. These glancing impacts on the concave airfoil surface can cause portions thereof to be eroded. This erosion typically occurs proximate or at or around the trailing edge nearest the tip of the airfoil. As a result, the airfoil steadily loses its effective surface area due to significant chord length loss, as well as becoming thinner, resulting in a decrease in compressor performance of the engine.
Hard coatings, such as titanium nitride and other nitride coatings, have been applied to the metal airfoil to alleviate or minimize such erosion. See, for example, U.S. Pat. No. 4,904,528 (Gupta et al), issued Feb. 27, 1990 (titanium nitride coating for turbine engine compressor components to reduce erosion); U.S. Pat. No. 4,839,245 (Sue et al), issued Jun. 13, 1989 (zirconium nitride coating for turbine blades to provide erosion resistance). However, standard titanium nitride coatings are less capable of resisting impact damage caused by particles that strike the leading edge of the airfoil directly or head on. Titanium nitride coatings can also adversely impact the high-cycle fatigue (HCF) strength of the airfoil. Thicker coatings, such as HVOF applied tungsten carbide coatings, can provide greater impact resistance than titanium nitride coatings. See, for example, U.S. Pat. No. 4,741,975 (Naik et al), issued May 3, 1988 (tungsten carbide or tungsten carbide/tungsten coating applied to a layer of palladium, platinum or nickel on a turbine compressor blade for erosion resistance). However, such tungsten carbide coatings are often too thick and heavy to be applied to fast rotating airfoils, especially for helicopter turboshaft engines, and are generally too thick to be implemented with existing airfoil designs.
Accordingly, it would also be desirable to improve the impact resistance properties of turbine airfoils, in particular turbine compressor airfoils used in helicopter turboshaft engines. It would also be desirable to be able to decrease erosion, and especially improve the erosion resistance of such turbine airfoils. It would be further desirable to improve such erosion resistance without adversely affecting other desirable properties of the turbine airfoil such as high-cycle fatigue strength.